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FIGURE 2.6-15 Results of a simulation of the model of Figure 2.6-14. Model neuron spikes are shown. Simulation was done using the 47-state, Simnon program, ARENcns3.t found in Appendix 1. The insect orientation angle, . , was assumed to be smoothly varied over 0° to 225°using a tilt of 45°. The horizontal axis of the gure is . . (Recall that . = 90° corresponds to pure roll right up; see the insects in Figure 2.6-5.) Part of the program modeled the cosine directional responses of the four VNC PI units driving the model. The bottom spike train is the simulated activity of the RCPI axon; the trace immediately above it shows the spikes on the RIPI axon during the change of tilt direction, . . The third trace up shows the spikes on the LIPI axon; the fourth trace up is the LCPI activity. The numbered traces are 1, output of the 45° PPU; 2, output of the 90° PPU (note that it res for . in the range of 80° to 120°, giving a sharp response); 3, output of the 135° PPU.° 4 to 7 show activities of interneurons 4 to 7, respectively. It is not surprising that 1 and 3 do not have sharp responses because the model is complete only around PPU 2.

To simplify the operation of the neural model, let . vary continuously and linearly from 0° to 225°, and plot the spike outputs of the seven neurons in the model as well as the spike frequencies associated with the IPFM models of the four VNC afferent units. A typical plot is shown in Figure 2.6-15. (A more accurate way to use the model would be to input x ed . values and measure steady-state pulse frequencies of the seven neurons.) The model parameters used were:

[ARENcns3] " Parameters 2/27/99 AREN6.t : maxF1:2, R:57.296, maxF2:1, phi2:3, tau:0.002, Do:0.5, Doo:0.5, Doi:0.5, co:1, phi:2, g2:5, g3:1, g3e:1, g6:0.3 , ci:1, ae:0.5, ai:0.5, pi:3.14159.

Note that the three output neurons re o ver a considerably narrower range of . than do the VNC units (80 to 120°); hence, the model demonstrates that sharpening of the positional response is possible, and it offers a possible mechanism responsible for this enhanced resolution.

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